In a layered protocol stack of a communication network the physical layer is responsible for coding, physical-layer hybrid-ARQ processing, modulation, multi-antenna processing, and mapping of the signal to the appropriate physical time-frequency resources. It also handles mapping of transport channels to physical channels. Within LTE (Long Time Evolution) the physical Downlink Control Channel (PDCCH) is used for downlink control information, mainly scheduling decisions, required for reception of PDSCH (Physical Downlink Shared Channel) and for scheduling grants enabling transmission on the PUSCH (Physical Uplink Shared CHannel). In particular, the PDCCH in LTE carries uplink grants and downlink assignments to the user equipment (UE) with relation to a particular eNodeB. The uplink grants allow a UE to transmit in the uplink (to the eNodeB) and a downlink assignment tells the UE that a downlink transmission is sent (from the eNodeB). Both the uplink grants and the downlink assignments are defined only for one Transmission Time Interval (TTI) e.g. 1 ms. For continuous transmission a new grant/assignment has to be sent every TTI. One exception to this is so called semi-persistent scheduling (SRS), where scheduling can be done for several TTI with one assignment or grant, specifying that the allocation is valid every x ms until it is inactivated by a new assignment or grant.
Typically the PDCCH is transmitted with so called QPSK (Quadrature Phase-Shift Keying) modulation and coding alternatives are available by allocating 1, 2, 4 or 8 Control Channel Elements (CCE:s) to each assignment/grant in PDCCH. The total number of CCE:s is limited and depends on the system bandwidth. If no link adaptation is made on PDCCH, 8 CCE:s must be allocated for all users to assure communication for cell edge users that experience bad channel quality.
Link adaptation for PDSCH is typically performed based on channel state information (CSI) which is reported from the UE. The CSI includes channel quality indicators (CQI:s) which guide the modulation and coding scheme (MCS) selection in the base station. Similarly, the number of CCEs to use on PDCCH could be based on the CQI reports, with a fixed offset value M to handle the mismatch between the channel that CQI meant to represent and the channel that PDCCH uses.
In FIG. 1 a typical signaling scheme for UE initiated UE setup is shown. The UE starts by transmitting a preamble on the Physical Random Access Channel (PRACH). The network node e.g. eNodeB responds to this with a Random Access (RA) Message 2 in which the UE also receives a grant for RA Message 3. In RA Message 3 the eNodeB receives the C-RNTI (Cell Radio-Network Temporary Identifier) which identifies the UE on a cell level. The eNodeB then knows that the UE is new in the cell and that an RRC connection Setup Request is needed to setup the UE. In the initial UE setup procedure this message is sent in RA Message 4, and the UE responds to this with a RRC Connection Setup Confirm in RA Message 5.
When a UE is new in the system the eNodeB has no or very limited information about the channel quality that the UE is experiencing. The channel quality for the uplink is typically characterized by the so called SINR (Signal to Interference and Noise Ratio) which includes both the wanted signal power and the interference power. In the uplink the interference is measured by the eNodeB receiver and is therefore available almost instantaneously, or the interference is known prior to the connection request and the eNodeB performs e.g. pathloss estimation for the particular UE. However, in the downlink the SINR is measured by the UE and explicitly signaled to the eNodeB using a Channel Quality Information (CQI) report. This means that until the eNodeB receives the first CQI report it has no knowledge about the SINR experienced by the UE. Typically, this is solved by using some very robust MCS until the downlink SINR is known. This is a robust solution but it has its disadvantages.
Amongst other things, the RA Message 2 is a very small message and can be transmitted with a low MCS on only a few PRBs with sufficient performance. However, RA Message 4 (RRC Connection Setup) can be significantly larger and using a small MCS for this (as in the example in FIG. 1) can require a lot of PRBs and also require segmentation into a number of separate downlink transmissions. Also, each of these transmissions require a PDCCH message that has to be encoded in a robust way, and will therefore use a large amount of PDCCH CCE:s.
On the other hand, if the initial downlink SINR is too high this will result in too aggressive link adaptation causing many retransmissions and potentially also a failed delivery of the message.
When the UE enter the system and stays active for a longer time, the loss due to this slow initialization is small. But in real networks UEs tend to have quite bursty traffic, where they enter the system, transmit and receive a little bit of data and then go back to idle again. Assuming this traffic model, the above described inefficient link adaptation that UE setups account for can be significant.
Based on the above, there is a need for a way to speed up the initialization of the downlink link adaptation, in particular for UEs with bursty traffic patterns.